J. Phys. Chem. 1960, 84, 1635-1638
1635
Organotini Polymers Formed by Glow-Discharge Polymerization Erlch Kny,+ Leonard L. Levenson,’ Wllllam J. James,** Departments of Physics and Chemistry and the Graduate Center for Materials Research, Universlty of Missouri-Roiia, Rolla, Missouri 6540 1
and Robert A. Auerbach Lord Corporation, Erie, Pennsylvanla 165 12 (Received October 15, 1979) Publication costs assisted by the Lord Corporation
Radio-frequency glow-discharge films were deposited on various substrates starting with tetramethyltin and monobutyltrivinyltin. The films obtained were amorphous and contain C, H, and Sn. The C/Sn ratio as determined by ESCA varies with the reactor parameters. C-enriched films are found to be usually transparent, whereas Sn-enriched films are metallic in appearance and are conductors. There is no apparent difference in the structure of the transparent and metallic films. The conductivity, composition, and adhesion, and surface morphology are reported. The findings favor the mechanism of the film-forming process as due to “atomic polymerization”.
Introduction The diverse application of organotin compounds as antifouling agents, biocides, catalysts, and stabilizers for olefinic polymers explains the increasing use and the expanding prolduction of such compounds. There has been a renewed interest in organotin polymers too. Such polymers are used mainly for the controlled release of bioactive compounds while simultaneously minimizing environmental hazards. The properties, use, and synthesis of organotin polymers are extensively reviewed by Subramanian and Garg.l In addition to the conventional synthesis of organotin polymers, there is another process available which yields polymer-like, organotin material in the form of thin films. Yasuda2 in his review on glow-discharge polymerization, points out tlhe exciting possibility of producing organometallic polymers by glow-discharge polymerization. The process of glow-discharge polymerization is different from conventional polymerization and yields different products. For lack of suitable substitutes, the expressions “polymerization” and “polymer” are still used. Yasuda also distinguishes between “molecular”, plasma-induced polymerization, and “atomic”, plasma-state polymerization. The former is essentially conventional polymerization initiated by a reactive species created in an electrical discharge. The starting monomer must contain polymerizable structures (unsaturated bonds). Plasma-state polymerization may be viewed as an “atomic” process whiclh takes place only in the plasma state. In the initiating step M M* a reactive species M* is produced from M which does not necessarily exhibit the structure or stoichiometry of the starting monomer, Le., M may be a fragment or an atam detached from the starting monomer. Plasma-induced polymerization can be represented by a conventional chain-propagation mechanism in which no gas-phase by-product is produced.2 In glow-discharge polymerization both mechanisms occur. The extent to which they occur is dependent upon the chemical structure of the starting material and on the condition of the discharge.
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Institut fur physicaliske Chemie, Universitat Wien, A-1090 Wien, Austria. t Department of Physics. $Department of Chemistry. 0022-3654/80/2084-1635$0 1 .OO/O
Tkachuk et ala3p4have reported that organotin polymer films prepared in a glow discharge can be used as insulating layers on microelectronic devices, as protective coatings, and as intermediate adhesive layers. Subsequent thermal treatment of these coatings can result in new films of very different properties. For example, tin oxide coatings can be prepared by pyrolysis of organotin polymer films. The pyrolysis of organotin coatings to tin oxides is the subject of a Japanese patent.6 The interface composition and the adhesion of organotin polymer films on A1 and stainless steel samples are described in detail in a recent paper by the authorsa6Another paper deals with the composition and stoichiometry of glow-discharge organotin films.7 A third paper describes the existence and the properties of metallic, conducting organotin films.8 It is the aim of this paper to summarize the authors’ present experience on production, composition, properties, and possible uses of glow-discharge organotin films. Although many properties of glow-discharge films are highly dependent upon reactor parameters, this paper emphasizes the reproducible features and general observations of glow-discharge organotin films. Experimental Section Synthesis of Glow-Discharge Organotin Films. Glowdischarge organotin films were prepared in an inductively coupled glow-discharge reactor (Figure 1). The central part of the reactor is a cylindrical Pyrex glass tube. The whole system is operated by means of a rotary pump in the millitorr range. The monomer gases or vapors are introduced by a leak valve at one end of the reactor at the desired flow rate. Since the pressure was recorded by a thermocouple gauge, the pressure readings can only be expected to be approximate. The starting materials were tetramethyltin (TMT) and monobutyltrivinyltin (MBTVT). The glow-discharge polymerization of TMT proceeds very readily and yields transparent, contiguous, pin-holefree films in most cases. Parameters used are given in Table I. At higher power settings, the polymerization yields brown, semitransparent films. The brown coloration of such films decreases quickly after exposure to oxygen (air). By introducing T M T into the reactor at a higher power setting, it was possible to obtain both semitransparent and semiconducting metallic films in different parts of the reactor. This is reported in detail in another 0 1980 American Chemical Society
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TABLE I: Parmetem for Glow-Discharge Experiment8 expt no. 21 starting compd TMT flow rate (STP),cm’ls 4.10 x rf D O W ~ I syitem pres., pg
duration, s
max depositn rate, n m h film appearance min. atomic C/Sn ratio obtained
Kny et ai.
1980
22
9
20 TMT 8.5 x 10.’ 8.9 x lo-’ 8.5 X lo-’ 1 7 w . 3 . 9 ~ ~ 1~ 7 w . 3 . 9 ~ ~i ~o w . 3 . 9 ~ ~3 ~7 w . 3 . 9 ~ ~ ~ 4-5.3 Pa 5-8 Pa 2.7-3.3 Pa 6.5-9.Pa 4450 4890 4900 3600 0.04 0.09 0.19 0.03 transparent transparent transparent metallic brown 2.8 3 3.5 1.2
TMT
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paper.8 There are as yet insufficient data available to yield graphs showing the interdependency of the main reactor parameters as deposition rate vs. power or mass flow rate vs. power, etc. It can only be stated qualitatively that for the T M T system the tendency to form brown and finally metallic deposita increases with increasing power. Transparent films are obtained a t low power. Increasing the flow rate a t constant power still yields a transparent polymer. The glow-discharge polymerization proceeds a t essentially ambient temperature. In the center of the glowdischarpe region the increase in the substrate temperature does not exceed 2 “C. When a vinyltin monomer of higher molecular weight is used as the starting material, the vapor pressure of the liquid is not sufficient to maintain the glow discharge. Heating of the monomer-containingvessel and of the inlet lines therefore are necessary. A t low flow rates and low power, a transparent polymer is obtained. A t increased flow rate and constant power, a white deposit is obtained. Examination by SEM reveals an agglomeration of spheres similar to those observed for the plasma-induced polymerization of ethylene?JO Characterization and Composition of Films. The submicrostructure of the films was determined by using standard equipment and procedures for obtaining SEM and TEM micrographs. The sheet conductivities were measured with a four-point probe? The solubilities of the f h were determined in a large range of solvents including water, alcohols, ketones, hydrocarbons, chlorobenzene, methylene chloride, and carbon tetrachloride. Adhesion to various substrates was effected by using a direct-pull method described in the literature.” The composition of the films was measured by using auger electron spectroscopy (AES). Calibration of the system was effected by using solidified TMT vapor as a standard. The calibration procedure is described elsewhere in detail!
Results a n d Discussion Examination of the TMT film by SEM revealed a smooth surface with no evidenced internal structure. Only a small number of spheres or none appear in the SEM micrographs. The morphology of films is essentially unchanged when the films are produced at higher power or
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Flgm 2. SEM microgtaphs of TMT and M B M Rhs d e p o s M in the central regbn of the radio-frequency coil: (A) MBTVT film on Si. experiment 9: (6) M B M film at Hgh Row rate of monomer; (C)TMT film on glass. d n i s identkxl with experiment 21; (D)TMT film on Si, experiment 22.
higher flow rates. The metallic films as expected exhibit a smooth featureleas surface. The glowdisehargeorganotin films are amorphousand show no structure when examined by TEM. These observations agree well with the concept of “atomic”, plasma-state polymerization as opposed to “molecular”, plasma-induced polymerization. TMT, because of its snturated bonds, has no possibility to polymerize in the conventional “molecular” manner. “Atomic polymerization” of plasma-created fragments may then be responsible for the formation of a solid with a smooth, featureless structure. On the other hand, MBTVT may polymerize conventionally (plasma-induced polymerization), as well as by plasma-state polymerization. The relative rates a t which these mechanisms proceed is dependent upon the particular reactor parameters used. The heterogeneous structure one observesgis a direct result of that dualism. SEM micrographs of plasma-polymerized MBTVT on different substrates reveal two-phase submicrostructures of spheres and a featureless film. The larger spheres which are agglomerates of smaller ones become more evident and populous as the flow rate is increased (Figure 2). AES and ESCA examination of the organotin films reveals that the stoichiometry with regard to the starting compound is not retained. AES or ESCA may be used to determine the C/Sn ratio. The C/Sn ratio is not constant throughout the reactor (Figure 3). Films high in tin content are deposited at the front end of the reactor,
The Journal of Physical Chemistry, Vol. 84, No. 12, 1980 1637
Organotin Polymers
in the pull area by ESCA and Auger analysis.6911 This implies that the adhesive force is in excess of the observed experimental value. The formation of tin oxide bonds between the substrate and the film on A1 samples, as evidenced by a shift in the ESCA Sn signal, suggests that the good adhesion is based on chemical rather than physical forces. The formation of tin oxide bonds was observed also on glass substrates.12 The high affinity of Sn to oxygen seems to be responsible for the bonding behavior and suggests a Sn-O-substrate linkage. Oxygen is ever present as an oxide layer on the Al. The presence of trapped radicals and distorted bonds13 accounts for the high surface activity for oxygen. The observation of the rapid disappearance of the brown color of glow-discharge films by oxygen is a first indication. The surface of the glow-discharge films are indeed saturated with oxygen as proven by ESCA (Figure 3). In contrast to the change in the C/Sn ratio, the Sn/O ratio remains constant throughout the reactor at a high level. This suggests that 0 is preferentially bonded to Sn at the very surface of the film. The shape of the AES Sn signal on the surface of organotin glow-discharge films is an additional indication that a tin-oxygen bond is formed.ll
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Figure 3. Atornic ratios of C/Sn and O/Sn vs. distance (in cm) from TMT inlet port (compare with Figure 1). The deposition rate is shown in nm/s. TMT film was deposited in experiment 21 (Table I).
whereas carbon-enriched films are deposited at the other end (tail flame region). The minimum observed C/Sn ratio was found to be lower than the C/Sn ratio of the starting T M T mono~ner.~” Similar findings were made with the MBTVT glow-discharge polymers, except that the C/Sn ratio remained at a higher level throughout. These findings also point tal the “atomic polymerization” mechanism as being dominant particularly in the case of TMT where the stoichiometric variations are very broad in range. Stoichiometric variations might also be attributed in part to the process of ablation. However, the effects of ablation are more pronounced for oxygen- and fluorine-containing compounds where there is a large amount of product gas formation. Where the main product gas is hydrogen the effects of ablation are minimal. Furthermore, in the region of highest visible glow (assuming that this corresponds to the highest flux density of plasma) we find no correlation with the observed minimum of the C/Sn ratio along the reactor axis. This suggests that radiation-induced desorption or ablation does not play a mlajor role in the broad stoichiometric departures from that of the starting monomers. It appears therefore that, in general, the organotin films form as molecular fragments and atoms (probably charged freeradical ions) combined in a random fashion to form amorphous, highly cross-linked polymer films. All glow-discharge organotin films were found to be completely iinsoluble at ambient temperature in the solvents previously cited. These findings are in agreement with the highly cross-linked structures attributed to plasma-formed polymers. The adhesion of organotin films formed from MBTVT was found to be surprisingly high on certain substrates. Values of up to 200 kg/cm2 were obtained on A1 by using the direct-pull test method. The failure mode, however, was cohesive ,asa thin film (
